Fuel cells convert gaseous fuels (such as hydrogen, natural gas and gasified coal) via an electrochemical process directly into electricity. A fuel cell continuously produces power when supplied with fuel and oxidant, normally air. A typical fuel cell consists of an electrolyte (ionic conductor, H+, O2−, CO32−, etc.) in contact with two electrodes (mainly electronic conductors). On shorting the cell through an external load, fuel oxidises at the anode resulting in the release of electrons which flow through the external load and reduce oxygen at the cathode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the cathode oxygen from the air or other oxidant is dissociated and converted to oxygen ions which migrate through the electrolyte membrane and react with the fuel at the anode/electrolyte interface. The voltage from a single cell under load conditions is in the vicinity of 0.6 to 1.0 V DC and current densities in the range 100 to 1000 mAcm−2 can be achieved.
Several different types of fuel cells have been proposed. Amongst these, the solid oxide fuel cell (SOFC) is regarded as the most efficient and versatile power generation system, in particular for dispersed power generation, with low pollution, high efficiency, high power density and fuel flexibility. SOFC's operate at elevated temperatures, for example 700-1000° C.
Numerous SOFC configurations are under development, including the tubular, the monolithic and the planar design. The planar or flat plate design is the most widely investigated. Single planar SOFCs are connected via interconnects or gas separators to form multi-cell units, sometimes termed fuel cell stacks. Gas flow paths are provided between the gas separators and respective electrodes, for example by providing gas flow channels in the gas separators. In a fuel cell stack the components—electrolyte/electrode laminates and gas separator plates—are fabricated individually and then stacked together. With this arrangement, external and internal co-flow, counter-flow and cross-flow manifolding options are possible for the gaseous fuel and oxidant.
Traditionally hydrogen, usually moistened with steam, has been used as a fuel cell fuel. However, in order to be economically viable the fuel must be as cheap as possible. One relatively cheap source of hydrogen is natural gas, primarily methane with a small proportion of heavy hydrocarbons (C2+). Natural gas is commonly converted to hydrogen in a steam reforming reaction, but the reaction is endothermic and, because of the stability of methane, requires a reforming temperature of at least about 650° C. for substantial conversion and a higher temperature for complete conversion.
One proposal for a fuel cell electricity generation process in which a hydrocarbon fuel is converted to a fuel cell fuel stream including hydrogen in a steam pre-reformer is disclosed in EP-A-0435724. The temperature in the pre-reformer is described as 700 to 850° C. with a resultant product-gas composition of 65-80 vol % H2, 5-20 vol % CO, and 5-25 vol % CO2.
Another such proposal is disclosed in U.S. Pat. No. 5,302,470 in which the steam pre-reforming reaction is said to be carried out under similar conditions to those of known steam reforming reactions: for example, an inlet temperature of about 450 to 650° C., an outlet temperature of about 650 to 900° C., and a pressure of about 0 to 10 kg/cm2.G to produce a fuel cell fuel stream which is composed mainly of hydrogen and is fed to the fuel cell anode via a carbon monoxide shift converter.
Hydrocarbon fuels suggested for use in the above two proposals include, in addition to natural gas, methanol, kerosene, naphtha, LPG and town gas.
While high temperature fuel cell systems produce heat which must be removed, heat exchangers capable of transferring thermal energy at the required level from the fuel cells to a steam reformer are expensive. Thus, hydrogen produced by steam reforming natural gas may not be a cheap source of fuel.
It has been proposed to alleviate the problem of the cost of substantially complete steam pre-reforming of methane by using natural gas as a fuel source for a high temperature planar fuel cell stack and subjecting the natural gas to steam reforming within the stack, at a temperature of at least about 650° C., using catalytically active anodes. However, this arrangement can lead to carbon deposition problems on the anode from C2+ hydrocarbons, and is not suited to other higher hydrocarbon fuels for this reason. Furthermore, given the endothermic nature of the methane steam reforming reaction, too much methane in the fuel stream can lead to excessive cooling of the fuel cell stack. To alleviate this problem the fuel stream has been restricted to a maximum of about 25% methane (on a wet basis) with the natural gas being subjected to partial steam pre-reforming at elevated temperatures approaching 700° C. upstream of the fuel cell stack.
Another process for producing electricity in a fuel cell from hydrocarbon fuels such as gasified coal, natural gas, propane, naphtha or other light hydrocarbons, kerosene, diesel or fuel oil is disclosed in EP-A-0673074. As described in that specification, the process involves steam pre-reforming approximately 5 to 20% of the hydrocarbon fuel at a temperature of at least 500° C. after start-up to convert ethane and higher hydrocarbons in that fraction to methane, hydrogen and oxides of carbon and to achieve a measure of methane pre-reforming in that fraction to oxides of carbon and hydrogen. Steam pre-reforming at this lower temperature alleviates carbon deposition in the pre-reformer. The hydrocarbon fuel with the steam pre-reformed fraction is then supplied to fuel inlet passages of the fuel cell stack which are coated with or contain a catalyst for steam reforming of the methane and remaining hydrocarbon fuel at 700-800° C. into hydrogen and oxides of carbon which are supplied to the anodes in the fuel cell stack.
Indirect internal steam reforming of the remaining hydrocarbon fuel within the fuel inlet passages in EP-A-0673074 is said to allow the use of reforming catalysts within the fuel inlet passages which are less likely to produce coking or carbon deposits from the internal steam reforming of the higher hydrocarbons than the nickel cermet traditionally used in anodes for solid oxide fuel cells. It is believed that steam pre-reforming of the hydrocarbon fuel in the described temperature range is restricted to 5 to 20% of the fuel in order to relatively increase the level of hydrogen in the fuel stream to the fuel cell stack and thereby alleviate carbon deposition when the fuel is internally reformed in the stack.
One of the advantages of reforming a hydrocarbon fuel stream including methane on a solid oxide fuel cell anode is the potential ability to thermally manage the fuel cell by appropriately balancing the exothermic fuel cell reaction resulting in the production of electricity with the endothermic methane steam reforming reaction. For optimum thermal management, the steam reforming reaction should occur immediately adjacent the fuel cell reaction so that the heat produced by the fuel cell reaction can be directly taken up by the steam reforming reaction. Although EP-A-0673074 describes the reforming catalyst as being in intimate thermal contact with the fuel cells, the contact in fact is by way of heat transfer through the tubes defining the passages in which the catalyst is disposed and further by way of a porous ceramic support, which would tend to hinder thermal management.
An alternative approach to providing a fuel stream for a fuel cell in which the proportion of methane derived from a higher carbon (C2+) hydrocarbon fuel is increased is disclosed in our International Patent Application No. WO 01/12452. In this proposal all the fuel is reacted with steam in a steam pre-reformer at a temperature in the pre-reformer of no greater than 500° C. to produce a fuel stream including hydrogen and no less than about 20% by volume methane (measured on a wet basis), with minimal, if any, C2+ hydrocarbons. The fuel stream is reacted at the anode of the fuel cell to steam reform the methane and to produce electricity when an oxidant such as air is reacted at the fuel cell cathode. A modification of this proposal is described in our International patent application PCT/AU02/00128.
In both these proposals, the proportion of methane in the fuel stream to a high temperature fuel cell in which the methane is internally reformed directly on the anode is increased, giving the potential for better thermal management of the fuel cell for the reasons described above. Any excessive cooling of the fuel cell due to the relatively high levels of methane being reformed can be resolved as described in WO 01/12452 and AU PR 3242. Carbon deposition resulting from steam reforming on the anode is alleviated by minimizing the proportion of C2+ hydrocarbons in the fuel stream contacting the anode. However, the risk of carbon deposition from methane in the fuel stream still remains.
Therefore, although the proposals in WO 01/12452 and PR 3242 do alleviate carbon deposition on the anode of a solid oxide fuel cell, it would be advantageous to further reduce the risk of carbon deposition from methane on the anode while maintaining the potential for good thermal management.